Magnetic focusing device



Sept. 19, 1961 SUSUMU YASUDA 3,001,094

MAGNETIC FOCUSING DEVICE Filed Jan. 7, 1960 LII A Home y 3,001,094 MAGNETIC FOCUSING DEVICE Susumu Yasuda, Tokyo, Japan, assignor to Nippon Electric Company, Limited, Tokyo, Japan, a corporation of Japan 7 Filed Jan. 7, 1960, Ser. No. 1,040 Claims priority, application Japan Jan; 14, 1959 1 Claim. (Cl. 313-84) In electron tubes such as travelling-wave tubes for which the diameter of electron beam is to be maintained approximately constant along a long transit section, a focusing magnetic field is generally required. By making the focusing magnetic field of a periodic magnetic field type, the focusing device can be made extremely compact and lightweight as compared with a uniform magnetic field that has been conventionally used. However, in waveguide coupling type travelling-wave tube amplifiers which have a waveguide at each of the input and output parts, discontinuities in the magnetic field are produced by the presence of these waveguides. This results in the electron currents diverging and being absorbed in the R.F. waveguide circuits. In order to solve this problem, the present invention comprises a method of utilizing a common magnetic flux of two magnetic lenses installed.

on opposite sides of the waveguide.

The invention will now be described in detail with reference to the appended drawing, wherein:

FIG. 1A shows a partial side view of the magnetic focusing device; 7

FIG. 1B shows a cross sectional view illustrating an embodiment of .the magnetic focusing device in accordance with the present invention; and

FIG. 2 illustrates a curve of magnetic flux versus the ratio of the diameters of the pole pieces to the cylindrical magnets.

As a result of performing theoretical experimentations and investigations (for example, reference will be made to A Proposal on production of a periodic field and its design method in the All-Japan Convention Record, page 187, 1958, Institute of Electrical Communication Engineers of Japan as well as the article entitled 7-GC band package type travelling-wave tube amplifier in the previously mentioned convention record, page 194), it has been admitted, referring to FIGS. 1A and 1B, that in this case the intensity of the magnetic field that can be produced is affected by the construction of the magnetic shunt circuit and that a strip-shape shorting plate 6 being magnetically coupled to pole pieces 4 and 5 of two cylindrical magnets 2 and 3 and so arranged on each side of the R.F. circuit such as waveguide 1 that magnetic poles of the opposite polarities face each other will result in the most desirable type of magnetic shunt. The stripshape construction of this type provides an efiective method of minimizing the confronting areas of the magnetic shunt circuit with respect to pole pieces 7 and 8 so as to minimize an increase in external leakage magnetic time A plurality of shorting plates may be used, although only a single plate is illustrated in FIGS. 1A and 1B. By sbort-circuiting is meant coupling of pole 3,001,094 Patented Sept. 19, 1961 pieces 4 and 5 through a suitable magnetic reluctance where the dimensions of the short-circuiting plate are so selected as to obtain a suitable magnetic saturation state by a shunt magnetic flux passing through 6.

A decreasein the above-mentioned uneffective leakage magnetic flux can also be accomplished bysuitably selecting the dimensions of pole pieces 4, 5, 7 and 8.

The present invention has been accomplished by a series of investigations based on experimental facts. It should be noted that by making the external diameter of the pole pieces smaller than that of magnets 2 and 3 in the present invention, the previously mentioned uneifective leakage magnetic flux can be decreased while the effective magnetic flux for focusing the electron beam can be increased.

FIG. 2 illustrates graphically the effect given to the maximum value of the periodic magnetic field .where the ratio of the external diameter 2R of the pole pieces to that of the magnets 211, is varied by using the shunting circuit of the same dimensions. As is indicated, the magnetic flux density is increased as much as 15 percent when the ratio of R2/R1 is equal to 0.88 instead of 1.00.

As has been publicly known, a strong magnetic field results in a strong focusing ability within the range of satisfying the condition necessary for focusing where an electron flow is. focused by a periodic magnetic field.

The focusing ability can be improved simply by making the diameter of the pole pieces smaller than that of the magnets and by making the construction of the magnetic shunt circuit discontinuous with respect to the circumferential direction such as illustrated as shunt 6 in FIG. 1A.

Thus the present invention is adapted for the construction of inexpensive, lightweight magnetic focusing equipment of high performance.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claim.

What is claimed is:

.A periodic magnetic focusing device for long electron beams comprising a pair of cylindrical magnets positioned with unlike poles adjacent one another, a wave guide disposed between said pair of cylindrical magnets, first pole pieces located at the extreme ends of said magnets, second pole pieces located between said magnets and said wave guide, the external diameter of said first and second pole pieces being smaller than the external diameter of the cylindrical magnets, and a magnetic shorting connection between said first pole pieces bridging across said cylindrical magnets and said wave guide.

References Cited in the file of this patent UNITED STATES PATENTS 2,812,470 Cook et al Nov. 5, 1957 2,855,537 Mendel Oct. 7, 1958 2,871,395 Ciofli Jan. 27, 1959 2,882,439 Nisllio et a1. Apr, 14, 1959 

